The present invention is generally directed to a system and method for displaying internal surface information. More particularly, the present invention is directed to a system and method for displaying internal surfaces existing at various depths within a three-dimensional body. The images of the surfaces displayed are typically contained within the interior regions of solid bodies which are examined by computer axial tomographic (CAT) x-ray systems or by nuclear magnetic resonance (NMR) imaging systems, either of which is capable of generating three-dimensional arrays of data representative of one or more physical properties at various locations within a three-dimensional object. The images generated in the practice of the present invention are particularly useful in that they provide three-dimensional data for examination by physicians, radiologists and other medical practitioners. The present invention is particularly useful in that it permits the simultaneous display of select tissue types on a display device. Even more particularly, the present invention is directed to a parallel, pipeline architecture for simultaneously determining surface type and for the generation of surface normal vectors for the display of shading characteristics.
In conventional x-ray systems, a two-dimensional shadow image is created based upon the different x-ray absorption characteristics of bone and soft tissues. A great improvement on the conventional x-ray system as a diagnostic tool is provided by computed axial tomographic systems, which have been developed over the last ten years or so. These so-called CAT systems are x-ray based and have been used to produce single two-dimensional views depicting transverse slices of a body, object or patient. Three-dimensional information was thereafter gleaned from CAT scan data by generating data for a number of contiguous slices and using the inferential abilities of the radiologist to suggest a three-dimensional representation for the various internal organs. In the present invention, shaded and contoured three-dimensional images are generated from the three-dimensional array of data generated by a sequence of such contiguous CAT scans or magnetic resonance imaging scans. The images are typically displayed on a two-dimensional screen, but the shading provided produces an illusion of a three-dimensional structure, just as in a conventional motion picture.
The newer magnetic resonance imaging technology possesses the capability to better discriminate between various tissue types, not just between bone and soft tissue and therefore offers the capability for producing more discriminating images in many situations. NMR imaging systems are also capable of generating physiological data rather than just image data. However, whether NMR or CAT systems are employed, data has generally been available only as a sequence of slices, and systems have not generally been available which provide shaded two-dimensional images which accurately depict three-dimensional views.
Prior work by at least one of the inventors herein has significantly solved some of the major problems associated with the production of high resolution, three-dimensional medical images. In particular, a system referred to as "dividing cubes" was disclosed in patent application Ser. No. 770,164 filed on Aug. 28, 1985 now U.S. Pat. No. 4,719,585, issued Jan. 12, 1988. At the time of the invention, all of the individuals in the present case and other related prior cases assigned to the same assignee, were under an obligation of assignment to the same assignee. The present application is also assigned to the same assignee.
Attention is now directed to the specific problem solved by the system of the present invention. In the display of three-dimensional images, and more particularly in the display of medical images, one often encounters three-dimensional objects having multiple interior surfaces which occur in layers at various depths. For example, three-dimensional data associated with physical measurements of the human head produce data associated with skin, bone (the skull), the brain, nasal cavities and various internal soft tissue structures. In a three-dimensional view of the head, for example, there are circumstances in which it would be desirable to be able to simultaneously display both brain and bone tissue structures. Likewise, there are situations in which it would be desirable to be able to quickly switch back and forth between views of skin and bone and/or brain tissue to reveal underlying structures and relationships between the structures. This ability is particularly useful prior to certain surgical procedures. While the dividing cubes system is capable of displaying selected tissues such as bone or skin or brain tissue, it is also nonetheless desirable to be able to display selected structures and/or to simultaneously display these structures on the screen simultaneously so as to more clearly indicate their relationship. This is particularly advantageous as a surgical planning method since it is capable of showing the relationship between various bodily structures. It is noted, however, that while the present invention is particularly directed to the medical imaging arts, there is nothing contained herein which would limit its use thereto. Any three-dimensional measurement process performed on an object having an internal structure is amenable to processing in accordance with the system of the present invention. All that is required is that measurements of physical properties be made so as to associate the physical property measurements within regularly spaced locations with the body being studied.
An image of the anatomy typically consists of the visible surfaces of tissues computed by scanning the data and projecting the surface patches onto a view plane. In a three-dimensional array of data, the volume element is called a "voxel", in analogy with that of the area element which is referred to as a "pixel" in two-dimensional situations. In certain systems, voxel size limits the resolution of three-dimensional reconstructions thereby resulting in images that appear block-like or stepped as compared to having the smooth surfaces of real tissues. Attempts to produce smoother images by averaging over neighboring voxels, however, actually tends to reduce the resolution of the images. Other methods for three-dimensional display generation of images have been based upon measurement of the distance from an imaginary observation point to a patch on the surface of the object and on the estimated surface normal of the patch.
To shade the surface of a three-dimensional image projected onto a plane, an intensity is calculated from the component of the unit normal vector which component is parallel to the view direction. Surfaces with normal vectors parallel to the view plane are fully illuminated, while those with normal vectors at oblique angles to the view plane are gray and surfaces with normal vectors perpendicular to the view plane are dark or black. The dividing cubes system estimates the surface normal direction from a gradient vector of the three-dimensional density function. This is a useful estimate since the gradient is perpendicular to surfaces of constant density. Consequently, the gradient vector is substantially parallel to the unit surface normal vector. The unit normal vector is calculated by normalizing the gradient vector at the surface of interest. In the dividing cubes system, the gradient vector defined at each lattice point is linearly or nonlinearly interpolated over the voxel to give a local value of the gradient vector at desired intermediate voxel locations. A unit surface normal is computed by dividing the gradient vector by its magnitude. The surface that results from the use of these interpolated normalized vectors appears smooth because the interpolated gradient vector continuously varies with distance across a voxel boundary. This form of gradient shading is preferably employed in the dividing cubes system in general and in the variation of the dividing cubes system.